DataGridView - Handle Conflict With Constraint In DataTable

I have a datagridview in which the user can add new rows. The datasource is a datatable who has a unique-constraint for the column "Position". Thus, if the user enters a position already existing, an exception is thrown. To avoid the ugly standard error message, I simply handle the dataerror-event and set e.cancel = true. This works. But if the user presses escape, the row_validated-event is triggered. I use this event to format the rows containing new data by changing the font color to red. Here another exception is thrown (SystemOutOfRange-Exception) telling me that the index xxx has no value. How can I achieve row-formatting for new/modified entries without getting this error?

Here is my code:
''' <summary>
''' Catch exception and cancel illegal action.
''' </summary>
''' <param name="sender"></param>
''' <param name="e"></param>
[Code] .....